As a fluid machine, there are, for example, types such as a hydro turbine, a pump and the like. For example, the fluid machine such as a Francis-type turbine has a rotating portion and a stationary portion, so that there occurs a leakage flow that is part of the working fluid flowing through the gap between them. The leakage flow does not perform energy exchange with a prime mover within the fluid machine, and a leakage loss is caused depending on a flow rate of the leakage flow. Therefore, to reduce the flow rate of the leakage flow as small as possible, a seal structure formed of a minimum gap is adopted in the vicinity of the inlet and outlet of the fluid machine.
In addition to the structure configured of only the minimum gap, this seal structure includes a structure that the passage is narrowed by forming a projection of a rectangular shape, a saw-tooth shape or a thread shape from, for example, a sealing surface of the stationary portion, at a part of the gap between the stationary portion and the rotating portion configuring a sealing portion, and a structure that the sealing portion is formed to have multiple stages.
The hydro turbine such as a Francis-type turbine uses river water containing earth and sand as a working fluid. Therefore, for example, a conventional seal structure that a passage is narrowed by forming a projection from a sealing surface of the stationary portion suffers from abrasion of the tip end of the projection due to river water containing earth and sand, resulting in a possibility that a sealing effect is deteriorated, and a flow rate of the leakage flow of the working fluid (river water) increases.